Formatting: Prevent wp_targeted_link_rel() adding an empty attribute.

Bypass adding a `rel` attribute when the `wp_targeted_link_rel` filter returns an empty string or other falsy result.

Props mcmwebsol, spartank, meatman89fs.
Fixes #45352.


git-svn-id: https://develop.svn.wordpress.org/trunk@44691 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/formatting.php

@@ -3065,6 +3065,11 @@ function wp_targeted_link_rel_callback( $matches ) {
 	 */
 	$rel = apply_filters( 'wp_targeted_link_rel', 'noopener noreferrer', $link_html );
 
+	// Avoid additional regex if the filter removes rel values.
+	if ( ! $rel ) {
+		return "<a $link_html>";
+	}
+
 	// Value with delimiters, spaces around are optional.
 	$attr_regex = '|rel\s*=\s*?(\\\\{0,1}["\'])(.*?)\\1|i';
 	preg_match( $attr_regex, $link_html, $rel_match );